a(n) = -1/16-3*n^2/8+17*n/12+n^3/12+(-1)^n/16.

Terms

a(0) =1a(1) =2a(2) =3a(3) =5a(4) =8a(5) =13a(6) =20a(7) =30a(8) =43a(9) =60a(10) =81a(11) =107a(12) =138a(13) =175a(14) =218a(15) =268a(16) =325a(17) =390a(18) =463a(19) =545a(20) =636a(21) =737a(22) =848a(23) =970a(24) =1103a(25) =1248a(26) =1405a(27) =1575a(28) =1758a(29) =1955